The invention relates generally to birefringent crystals and filters and, more particularly, to a composite birefringent crystal and a folded birefringent filter using such a composite birefringent crystal.
The birefringent filter of FIG. 1(a,b) is a construct well-known to astronomers since the 1930s. Lyot [1] and Evans [2] used such filters in combination with their telescopes to image the sun within narrow frequency bands, for example isolating the helium line to provide for the imaging of helium gas dynamics on the sun""s surface. (Note in this specification, a reference to another document is designated by a number in brackets to identify its location in a list of references found in the Appendix) The birefringent filter appears to be introduced to optical telecommunications in the late 1980s by C. Burher, who demonstrated a simple periodic filter [3, 4, 5]. In the late 1990s the birefringent filter has become a lead contender for the attractive operation of interleaving and de-interleaving DWDM wavelengths.
A birefringent filter proper, 110 or 111, rotates the state of polarization (SOP), 106, at the output, 105, with respect to the input, 103, as a function of the optical frequency. The frequency response of the induced polarization rotation is tailored by the relative orientation of the ordinary birefringent axes, 109, from one birefringent element to the next. The relative orientations can be calculated using filter-synthesis procedures as described in Harris [6]. Advantageous filter designs can be realized by use of all birefringent elements having the same length, e.g. 110, or all birefringent elements having integral-multiple lengths of a unit length, e.g. 111.
In order to generate an amplitude response from the polarization rotation produced by a birefringent filter proper, Lyot and Evans both added input and output polarizers, 120 and 121. In this manner, input light 101 with input SOP 102 is first polarized to a linear state, 104. Birefringent-filter proper 110 or 111 receives linearly polarized input beam 103 and produces in general an elliptically polarized beam 105 with output SOP 106. Output polarizer 121 then analyzes the SOP 106 and produces linearly polarized beam 107 with SOP 108. As the optical frequency of input beam 101 changes, the output SOP 106 changes, producing an amplitude change of beam 107.
The use of polarizers for an optical telecommunications application is generally disadvantageous because of the polarization-dependent loss that results. Buhrer proposed and demonstrated the substitution of input and output polarizers with a polarization-diversity scheme. FIG. 2 illustratively shows the use of input polarization diversity 201 and output polarization diversity 202 elements in place of polarizers. Buhler""s U.S. Pat. No. 4,987,567 [4] provides one such architecture. The above-referenced Damask and Doerr patents, Ser. Nos. 09/532,143 and 09/532,150, cover alternative schemes.
FIG. 2 illustrates input beam 101 with SOP 102 being split into two parallel yet offset beams 210 and 211 by input polarization diversity element 201. Beams 210 and 211 have polarizations 220 and 221 that may be either orthogonal or parallel, depending on the method of polarization diversity implemented. The clear aperture of the birefringent filter proper, 110, is designed large enough to accept both beams 210 and 211. Beams 212 and 213 are output from the filter with polarizations 222 and 223, which may be orthogonal or parallel, depending on the method of polarization diversity implemented. Output polarization diversity element 202 then combines the orthogonal polarization elements of each SOP, 222 and 223, creating beams 214 and 215 with SOPs 224 and 225. In this scheme, no optical power is in principle lost. The intensities of beams 214 and 215 alternate as a function of input optical frequency such that the sum of their optical powers remains constant.
Whether input and output polarizers or polarization diversity elements are used to implement the transformation from SOP rotation to amplitude response, it is the core birefringent filter 110 which dictates the shape and periodicity of the frequency response of the filter. The relative orientations of the birefringent ordinary axes dictate the filter magnitude and phase response; the thickness of the birefringent plates dictates the periodicity of the response. The number of birefringent elements required to realize a specific filter shape dependents on the filter specifics. Typically, though, three or more stages are used.
As a practical matter, the unit crystal length of any one birefringent element, Lo, is on the order of 17.65 mm to achieve a frequency periodicity, called the free-spectral range (FSR), of 100 GHz using calcite as the birefringent material. A three-stage filter is then 52.95 mm long. Many important filters require more stages. Accordingly, as a practical matter, the length and material cost of the birefringent filter proper can be large. Moreover, the length tolerance from one birefringent element to the next is stringent. To shift the response of any one birefringent element by one FSR, the crystal length need change 100 GHz/193 THz, or about 0.05%, assuming an approximate and illustrative 1545 nm optical wavelength. Therefore the length control for calcite elements must be a small fraction of 0.05% of 17.65 mm, or less then about 9 microns. Typically crystal length can be controlled to about +/xe2x88x923 microns. The impact of small variations of crystal length along a birefringent filter is to distort the desired spectral response.
What is needed is a birefringent filter design that overcomes the above limitations of existing designs.
In accordance with the present invention, the problems of prior birefringent filter designs are overcome using a folded birefringent filter incorporating a multi-pass architecture. More generally, I have invented a novel composite birefringent crystal which includes a stack of two or more birefringent crystals having complimentary properties arranged to have zero net beam walk-off at off-normal beam incidence and a finite free-spectral range. In another embodiment, the birefringent crystal materials are selected to provide reduced temperature dependence. The result is an optically and mechanically stable composite birefringent crystal. In one application, a folded birefringent filter is implemented so that an input beam has multiple transits of the composite birefringent crystal. The folded birefringent filter uses the composite birefringent crystal together with one or more highly reflective devices and waveplates arrays to form a variety of single- or multiple-order folded birefringent filters.
More particularly, my composite birefringent crystal comprises:
a. a stack of two or more uniaxial birefringent crystals, each having front and back substantively parallel surfaces in which plane the crystalline extraordinary axis substantively lies and each located one behind the next with surfaces parallel, which receives an input optical beam that is non-normal to the front surface of the first crystal and produces from the back surface of the last crystal first and second orthogonally polarized optical beams;
b. where at least one crystal in the stack exhibits positive uniaxial birefringence;
c. where at least one crystal in the stack exhibits negative uniaxial birefringence;
d. where the extraordinary axis of at least one of the positive uniaxial crystals and the extraordinary axis of at least one of the negative uniaxial crystals have non-parallel alignment;
e. where with respect to the length of the first crystal, the ratio of the length of each subsequent crystal to the length of the first crystal is selected to produce at the bottom surface of the last crystal
i. zero net spatial displacement between first and second orthogonally polarized optical beams, and
ii. temporal delay between first and second orthogonally polarized optical beams.